When Perry arrives at Selkirk's place with Claire, the shot of Mason's car shows that Perry is alone in the car. In a later scene with Della, Perry is driving a different, newer model car. The scene with Della was filmed for this episode while the earlier scene was taken from the first season episode "The Case of the Vagabond Vixen" which explains the absence of Claire and the older car.
Perry: How would you like to pose as my wife?
Della: During working hours?
Perry: Do you want a bonus?
Della: Don't sell yourself short.
The ending and culprit in this episode differ from that of the novel on which it is based.